In the contemporary mobile phone business field, the introduction of super-small radio base stations for specified users are being considered and proposed so as to achieve “FMS (Fixed Mobile Substation) service” aiming to gain profitability from the fixed phone market by prompting users to use mobile phones in place of fixed phones.
Cells managed by such private, super-small radio base stations for specified users are collectively called “femtocells.”
To avoid interference with a cell managed by a general radio base station constituting an existing public communication network (referred to as a “macro cell” for convenience), the femtocell is sometimes operated at a frequency different from that of the macro cell.
In a mobile communication system using a WCDMA scheme, a mobile station using a macro cell does not measure control signals of other frequencies unless the reception quality of all the control signals of a currently-used frequency becomes or falls below a certain value.
This brings about a problem that the mobile station cannot find the femtocell and therefore cannot use the femtocell even when it is better for the mobile station to use preferentially the femtocell for such reasons as performing high-speed communication and receiving a better charge rate.
Several methods are under consideration as countermeasures for such a problem.
One of those methods known is to differentiate between a “PLMN-ID (Public Land Mobile Network-Identifier)” for a femtocell and a “PLMN-ID” for a macro cell.
In this method, the femtocell “PLMN-ID” is given higher priority than the macrocell “PLMN-ID,” and a mobile station finds the femtocell by periodically measuring control signals of the frequency of the femtocell.
However, in order for the mobile station to speedily find the femtocell, the interval for measuring the frequency of the femtocell needs to be short. This leads to a problem of power consumption by the mobile station.
Further, in another method under consideration, the mobile station itself determines the timing for measuring the control signals of the femtocell based on the IDs of a macro cell and a location registration area, respectively, in both of which the mobile station is located.
In this method, the mobile station stores therein the IDs of a macro cell and a location registration area that overlay a femtocell to which the mobile station is authorized to connect. Then, the mobile station measures the control signals of the frequency of the femtocell when a macrocell ID and a location registration area ID broadcasted by the macrocell match the macro cell ID and the location registration area ID that are stored in the mobile station.
In this way, the area for which the mobile station measures the control signals of the frequency of the femtocell is limited; therefore, the problem of power consumption by the mobile station can be solved.